1. Field of the Invention
The present invention is related generally to preparation of copper-indium-diselenide CuInSe.sub.2 crystals, and more specifically to an improved method for preparing stochiometric high quality CuInSe.sub.2 crystals in an open-topped crucible under pressure.
2. Description of the Prior Art
Copper-indium-diselenide (CuInSe.sub.2) crystals are of interest in the semiconductor industry as potential semiconductor material, especially for heterojunction type semiconductors, although the full extent of their potential is not presently known. Presently known processes for producing CuInSe.sub.2 crystals are cumbersome and totally inadequate for any efficient, large-scale production. Consequently, research and development efforts relating to testing and potential uses of CuInSe.sub.2 crystals are inhibited by excessive cost and lack of readily available crystals.
In order to produce the crystals, copper (Cu), indium (In), and selenium (Se) have to mixed together at a high temperature of approximately 1000.degree. C. to 1100.degree. C., which is just above the 986.degree. C. melting point of CuInSe.sub.2. One of the significant problems is that Se becomes volatile and begins to vaporize quite rapidly at about 700.degree. C. to 750.degree. C., and it has a very high vapor pressure. Therefore, something has to be done to control the Se vapor and hold it in the presence of the Cu/In melt for a sufficient period of time during synthesis of the compound CuInSe.sub.2.
The conventional method of preparing CuInSe.sub.2 crystals involves a synthesis of the Cu, In, Se elements in a melt that occurs inside a vacuum sealed tube. The process essentially involves melting a mixture of Cu and In and exposing this melt to Se vapor for a sufficient period of time to allow a synthesis of these elements, and then cooling the melt to grow the crystals, all of which occurs in the vacuum sealed tube.
More specifically, chunks of the desired amounts of Cu and In are mixed in an elongated quartz crucible or "boat", and the crucible is positioned near the closed end inside an elongated quartz tube. A semiporous plug is then positioned in the tube to separate the interior of the tube into two chambers with the crucible of Cu and In in one chamber. The desired quantity of Se is measured into a quartz flask, which is then placed into the second chamber of the tube. The tube is then evacuated and sealed, usually by melting the open end of the tube and pulling the softened quartz closed.
Finally, heating coils are positioned around the tube. There is preferably one heating coil around the portion of the tube that houses the first chamber and contains the crucible of Cu and In, and a second, separately controlled, heating coil is positioned around the portion of the tube that houses the second chamber and contains the flask of Se. However, even though it would be desirable to be able to heat the crucible and the flask independent of each other, their close proximity makes that feat virtually impossible. Therefore, the Se usually vaporizes before the Cu and In reach their melting points. However, the semiporous plug between the crucible and flask tends to retard the migration of Se from the flask to the crucible until the vapor pressure increases sufficiently to force the Se through the plug. Then, the semiporous plug tends to feed the Se vapor slowly to the crucible where it synthesizes with the Cu/In melt.
The melt temperature is held for a period of time, usually in the range of one to ten hours, in order to complete the synthesis. Then, the elongated crucible is preferably cooled slowly from one end to the other in an attempt to grow the CuInSe.sub.2 crystal from one end of the crucible to the other. Such preferential cooling can be attempted by sliding the tube longitudinally in relation to the heating coil.
While this conventional method is somewhat effective for preparation of semicrystalline growth, it is quite crude, labor intensive, and not very conducive to quality control, particularly of preferential cooling. Further, there is no convenient way to insert a seed crystal into the melt to grow a single crystal either by slow cooling or by Czochralski pulling. Therefore, it is practically impossible to obtain a large single crystal of CuInSe.sub.2 with this method.
Sometimes a larger single crystal can be obtained by breaking the tube to recover the polycrystalline CuInSe.sub.2 from the crucible, placing it in a different, second elongated vertical crucible, placing this second crucible in a vertical quartz tube, evacuating and sealing the tube, and heating the polycrystalline CuInSe.sub.2 to a melt. Then the melt is preferantially cooled from the bottom to the top, which sometimes results in growth of a fairly large single crystal. Even this extra step, however, does not always result in growth of a single large crystal. Regional temperature control precise enough to preferentially cool from the bottom of the melt to the top is still a problem, and sometimes reaction between the melt and the crucible prevents single crystal growth. The high vapor pressure of the Se also causes this method to be somewhat dangerous in that these closed tubes or ampoules are subject to breakage.
In 1962, a technique for pulling single crystals from a melt of PbTe and PbSe covered with molten B.sub.2 O.sub.3 in a relatively low pressure environment was published by E. P. A. Metz, R. C. Miller, and R. Mazelsky in the Journal of Applied Physics, Vol. 33, No. 6, p. 2016. This technique was advanced to crystal pulling in high pressures by J. B. Mullin, R. J. Heritage, C. H. Holliday, and B. W. Straughan, as published in 1968 in the Journal of Crystal Growth, Vol. 3, No 4, p. 281, where it was used to pull InP and GaP crystals. Pressures in the range of 25 to 40 atmospheres were used. However, those authors reported problems with contamination of the crystalline structures by the crucible materials made of silica and vitreous carbon. They also reported problems with boron contamination from the B.sub.2 O.sub.3 encapsulating material.
CuInSe.sub.2 crystals present even more difficult problems because Se has such a high vapor pressure or volatility, and the materials are very reactive. Further, CuInSe.sub.2 has complex phase ordering transitions in the cooling and crystal growth process that are not present for the crystals discussed above. These extreme characteristics of CuInSe.sub.2 have caused persons skilled in this art to be quite skeptical of the applicability of the liquid encapsulation techniques described above and to generally discount those techniques for any CuInSe.sub.2 crystal growth process. Consequently, use of the conventional vacuum sealed ampoules for synthesizing CuInSe.sub.2 crystals has persisted to the present time, in spite of the disadvantages described above.